From a bulk and weight point of view, I can't imagine why you'd ever want combine two sleeping bags to make a warmer bag.

because if your on a budget and its what you have ... going out and using a system that works but is a pound or so heavier beats sitting on the couch, eating cheezy poofs and doing virtual gear testing on BPL anyday ;)

RE OP - I'm not sure a single bag is ALWAYS best even if you have limitless budget. Some people (eg andy kirkpatrick http://www.andy-kirkpatrick.com/articles/view/double_up) swear by it due to the humidity in the synth thingi

PHD (in the UK) have done a few down combis bag for a bit now. They're just standard ones cut a little bit wider/longer and there isn't any clear reason that a quilt should do the same job fairly well.

Their basic pitch for their main series is that you have that and a very light summer bag, use the very light bag in Summer, the combi in Spring/Autumn and the pair in winter. As well as the cost the whole thing is very pleasingly 'tidy' from a conceptual point of view. The 'cost' seems to be ~100g in Autumn/Spring (wider bag so heavier) and maybe 200-250g in winter. Seems a very reasonable trade off to me.

They also do a combi bag for their very warm series so with two data points can get a, very rough!, ball park figure for combination. The ultralight series is +5,-5 individually and -15 when combined. The warmer stuff is -9 and -6 going together to -33. These are rough figures of course, but they'll be honest estimates.

The combination rule seems to be something like combined difference from +16 :) That's with two bags fitting perfectly together of course. Even a quilt over a normal bag probably won't be quite as effective overall and two normal bags might be a lot worse.

I am a frequent outdoor camper in the winter, but only in the Northeast, where below zero is less common.

So, I think I'm going to try to augment my EMS Solstice 20º with a TNF Aleutian 55º and see how it works. I want the Aleutian for the summer anyways, since it weighs very little in comparison to bags that cost more than 2x as much

($80 Aleutian weighs 1lb 12oz versus the $210 Sierra Designs Wicked Hot 1lb 10oz. The Wicked Hot is 45º instead of 55º but I bet the difference is negligible enough, since the TNF one has a hood and is less roomy.)

I use two down bags quite a bit. Works fine for me, just mix and match according to the expected temperatures. I use Western Mountaineering's Sleeping Bag Comparison Chart to estimate how much down is needed. (and also just for window shopping)

Examining the chart infers that an estimate for the amount of down required at an expected temperature is given by: Ounces = (55-T)/2 where T is the expected temp in degrees Fahrenheit. That is, for 30f you would need (55-30)/2 = 12.5 oz down. For -40 f that would be:(55-(-40))/2 = 95/2 = 47.5 oz.

Basically, an ounce of down is worth 2 degrees Fahrenheit. So mixing a 40 f synthetic bag with a 12 oz down bag would give you 40-12(2)= 16 f.

Inherent in this estimate is the assumption that the down is working fairly efficiently and is not overly compressed or overly loose fitting.

Like others have suggested, I do a lot of backyard testing when a cold spell comes to town. Try it, the neighbors probably suspect you're a nut anyway.